This invention relates to the field of electric machines, and more particularly, electric machines for automotive vehicles.
Electric machines are important components of conventional internal combustion engine automobiles. For example, electric machines typically serve as starting motors to crank automobile engines. Other electric machines serve as alternators that generate electricity from engine motion and deliver power to automobile loads.
Electric machines are also very important in modern hybrid electric vehicles (HEVs). HEVs combine an internal combustion engine with an electric drive system powered by a battery bank. In these hybrid vehicles, electric machines are typically required to operate as (a) a starter motor, (b) an electric drive and drive assist (i.e., propulsion and propulsion boost), (c) a generator which provides electric power for onboard electric loads and charges the battery banks, and (d) a re-generator which converts kinetic energy from the vehicle to electric power for charging the battery bank during braking/deceleration of the vehicle.
Hybrid Electric Vehicles can operate with low fuel consumption and low air-pollution. There are two propulsion systems onboard the HEV: (i) the traditional diesel or gasoline engine, and (ii) the electric drive system. The additional electric drive system consists of an energy storage compartment in the form of a battery bank, control components in the form of a power electronics unit, and an electric machine conversion component operable to convert electrical energy to mechanical energy and vice-versa. Thus, the electric drive system provides engine cranking, propulsion, power generation and power regeneration.
The electric machine is a core component in the HEV's electric drive system. The electric machine will run under the motoring state during vehicle starting, during pure electric drive and during electric assist drive. The electric machine is required to operate under the normal generating state during engine drive (thereby charging batteries) and the re-generating state during vehicle braking. Of course, the efficiency of the electric machine will directly govern the efficiency of electric drive system and consequently the fuel economy of the vehicle.
As the power conversion component in an electric drive system, the electric machine interacts directly or indirectly with the drive shaft or engine shaft and is located “under the hood” of the vehicle. However, the space available for the electric machine in the required “under the hood” location is typically limited. Therefore, the size and dimensions of the electric machine must be as small as possible. Compared to other applications for electric machines, high efficiency and small size are more important to the electric machine onboard the HEV. Furthermore, automobile manufacturers are increasingly calling for high efficiency and small size for almost all electrical machine applications in vehicles. Therefore, the need for small and mid-sized electric machines having high efficiency and small size is applicable to all automotive vehicles, and is particularly applicable to HEVs and purely electric vehicles.
One way to reduce the size and increase the efficiency of an electric machine is to increase the slot-fill-ratio (SFR) of the electric machine. SFR is typically defined as the ratio of (a) the aggregate cross-sectional area of bare copper conductors in a slot to (b) the cross-sectional area of the slot itself. With a high SFR, the large cross-sectional area of the copper wires helps reduce the phase resistance and consequently the resistance of the windings (i.e., power loss) for a given slot size, so the efficiency of the machine is improved. Accordingly, a high SFR allows more efficient electric machines to be built at a smaller size than less efficient predecessors.
Armature windings of most small and mid-sized electric machines are typically wound in many turns with single or multiple strands of round conductors in the form of round wires. FIG. 7A shows an exemplary prior art stator slot having a plurality of round conductors in the stator slot. The SFR in these machines with round conductors is typically between 35% and 45%. This relatively low SFR causes either low efficiency for given machine size or large package dimension for given performance requirement.
In the automotive industry, marine industry, and aerospace industry, the machine package size and efficiency have become very important, because available on-board space and fuel economics are critical requirements for machines in these industries. Reducing winding resistance by increasing SFR is one important strategy that may be used to improve torque density and efficiency for electrical machines with limited machine dimensions. In the past, rectangular conductors, such as those shown in FIG. 7B have been used in many of these machines to increase SFR and consequently lower winding resistance. These rectangular conductors are typically bar-shaped conductors consisting of either single or multiple strands of wire having a rectangular cross-sectional shape. In some machines, the bar-shaped wires are pre-shaped before the wires are placed into the stator slots. However, unlike thin strands of round wires, pre-shaped continuous bar wound windings cannot be laid into semi-closed slots, which are often preferred as discussed below.
To insert pre-shaped continuous bar windings into the slots of a stator, open slots are necessary, such as those shown in FIG. 7B. The openings in these slots allow the continuous windings to be inserted from the inner diameter of the core. However, closed or semi-closed slots, such as those shown in FIGS. 7C and 7D are typically preferred, as these slots can minimize winding vibration and damage that would otherwise occur in an open slot. Furthermore, semi-closed slots reduce equivalent air-gap length and harmonic flux density, as is known to those of skill in the art. Slot wedges, such as those shown in FIG. 7C, are sometimes used to provide the operational advantages of closed or semi-closed slots with the manufacturing advantages of open slots. These slot wedges include a wedge of magnetic material inserted in the open end of the slot.
To achieve the high SFR benefits of rectangular bar-shaped conductors and the operational benefits of semi-closed or closed slots, the U-shaped segmented conductors (also referred to herein as “hairpins” or “U-shaped bars”) of rectangular cross-section have been used in the past. A typical application of hairpin windings was 50DN alternator produced by Remy International since the middle of the last century. A cross-sectional view of the rectangular conductors in the core of the 50DN machine is shown in FIG. 7D. In the 50DN machine, the conductor wire is first cut into many segments with each segment having a certain length. The straight segments of wire are then bent and twisted into U-shaped conductors (or “hairpin” conductors) with the proper span. The hairpin conductors are inserted into closed or semi-closed slots from an insertion end of the stator. After the hairpin conductors are inserted into slots with slot liners, the legs of the hairpin conductors extend from a connection end of the stator. These legs are then bent to appropriate positions. Finally the proper leg ends are connected together to complete the windings. These connections include adjacent leg ends that are aligned directly and welded together or non-adjacent leg ends that are connected through jumper wires. Together, the connected conductors form the complete armature windings.
When large conductors, either round or rectangular shape, are used as armature windings, AC resistance in the windings increases with conductor size because of the phenomena known as skin effect. This resistance is experienced especially in the dimension of the slot height direction. The increment of AC resistance due to skin and approximation effect of larger conductors becomes even more pronounced at high frequency. Therefore, the conductor size, especially the dimension in the slot height direction, has to be limited in order to reduce AC resistance incremental losses in electrical machines. Lowering AC resistance losses is a principal strategy in designing electric machines with more poles and high efficiency at high speed. Reduction of conductor height/thickness in the slot produces more conductors in the slot height direction. For example, the traditional one set of windings (i.e., two conductors per slot, such as that shown in FIG. 7D) is replaced by two or more sets of windings in which there are four, six or even more conductor in slot height direction (see, e.g., FIGS. 5A-5C). The four or more conductors in a slot can be arranged in various winding set configurations. For example, the four conductors in a slot can be arranged as follows:                (a) one U-hairpin over another (see FIG. 5A and the corresponding stator of FIG. 6A);        (b) one U-hairpin crossing another (see FIG. 5B and the corresponding stator of FIG. 6B); and        (c) one U-hairpin side-by-side with another (see FIG. 5C and the corresponding stator of FIG. 6C).        
One common characteristic for the above arrangements with multi-set hairpins inserted into a core with a number of slots is that the slots per pole per phase (q) is an integer, i.e., q=integer.
Another common characteristic of hairpin wound electric machines is that they often are wound using wave windings. While lap windings can provide more parallel paths, more jumping connections are required at the end-turn region of the machine. These jumping connections are undesirable because they either crowd the end area of the core and increase the potential for short circuits, or require additional space that increases the package size. To reduce the jumpers or connection wires, a wave wound strategy is applied in most segmented U-shape bar windings. Generally, wave wound windings provide one or two parallel paths per phase in real production.
The number of turns in series per phase (i.e., Nph) is used to adapt the system voltage and torque for a given pole number and package size of an electric machine. For multi-set rectangular hairpin windings, Nph can be expressed asNph=2pqS/a  (equation 1)
where p=Number of pole pairs;
q=Slots per pole per phase;
S=Number of winding sets; and
a=Number of parallel paths per phase, which is 1 or 2 for wave windings and any integer number of 2p/a for lap windings.
For example, for a three phase machine having 2 winding sets (S=2), 12 poles (p=6), and wave windings with one parallel path per phase (a=1), the following series turns per phase is calculated:Nph=2pqS/a=(2)·(6)·q·(2)/(1)=24q Obviously, in this example, the series turns per phase winding can only vary with the integer times 12, if q=integer. In other words, Nph=24, 48, 72, 96, etc. However, in practice, q cannot be too large because of physical limitations on the total number of armature slots. Variation of pole pairs p could be used to obtain a different series turns per phase value, Nph, but this is still very limited.
Therefore, it would be advantageous to provide an electric machine operable for use in the automotive industry, marine industry, and aerospace industry, where the slots per pole per phase of the electric machine is not limited to an integer number such that numerous different series turns per phase may be achieved. Specifically, it would be desirable to provide an electric machine winding where an optimal number of turns in series per phase is provided such that the electric machine is operable to deliver the desired system voltage and torque for a given pole number and package size.